1. Field of the Invention
The present invention relates to electrode designs, excitation methods, and process conditions for the in-line, continuous treatment of flexible webs and films for improved surface properties, and to the resulting films and webs having modified surface properties.
2. Description of the Related Art
The applications and demands for improved surface modifications of polymer films and webs have grown considerably in the past ten years. Increasing environmental concerns are imposing tighter restrictions on the levels of solvents, adhesives, and surfactants often used with polymer films and webs. Plasma surface treatment offers an alternative to the use of these chemicals as well as polymer additives and the caustic chemical treatments often used for the modification of polymer surfaces. For many industrial applications, high speed in-line treatment of a film or web substrate is the desired objective. Treater equipment having small floor signature and the capability of being interfaced to existing production line equipment is often a necessity.
The high-volume plasma treatment of polymer webs or films has traditionally been done using either corona treatment at atmospheric pressure or low-pressure (P.ltoreq.1 Torr) discharges in batch mode operation. Conventional corona treatment using air has been around for at least 40 years and provides an economical but limited technique for surface modification. Altering the working gas of a corona treater provides a first step in expanding the utility of conventional corona discharge technology. Commercial corona treaters typically operate using low-frequency sinusoidal or half-sine-wave excitation in the frequency range of 60 Hz to 30 kHz. The electrode designs and excitation methods used tend to limit the discharge power densities and duty cycles attainable.
U.S. Pat. No. 5,576,076 by Slootman et al. discloses a process for depositing a layer of silicon oxide on a traveling substrate using silane gases at high pressures, typically atmospheric pressure. The electrode structure is designed so that an inert gas (preferably nitrogen) is introduced upstream of a moving substrate. By using a combination of suction and sufficient inert gas inlet flow, the introduction of ambient air into a discharge region can be controlled. A small amount of a silane gas and oxidizer along with an inert buffer gas are introduced into a corona-type discharge zone. Bar-type electrodes are excited using a sinusoidal signal at frequencies up to 60 kHz. The discharge is typically operated in a filamentary mode, requiring a dielectric barrier on the bar electrodes. The process yields surface treatments considerably improved over conventional corona treatments in terms of increased hydrophilicity and lamination strength. The process as disclosed however uses significant amounts of the inert buffer gas. The hydride silane (SiH.sub.4) and an oxidant are the preferred functionalizing gases and are about 1% to about 4% of the inert gas. Special precautions must be taken in gas handling since this silane is pyrophoric and expensive.
U.S. Pat. No. 5,527,629 by Gastinger et al. is a process patent similar to U.S. Pat. No. 5,576,076 with the added step of introducing a silane gas concomitantly or after plasma treatment. A low-frequency (20 kHz) dielectric-barrier corona-type discharge is also employed. Both of these treatment processes could benefit from the use of a more uniform plasma to produce improved surface uniformity, and reduced-pressure operation to decrease gas consumption.
Low-pressure systems, which typically operate around 1 Torr or less, have the versatility of using essentially any gas or volatile substance as a plasma medium; however, not all low pressure discharge methods scale well or are capable of generating a high-power-density discharge. In order to avoid some of the process restrictions of low-pressure batch processes, some recent applications of low-pressure discharges utilize annular gaps with differential pumping or roller seals to generate a vacuum interface. These low-pressure continuous-feed systems typically operate at or below 1 Torr, require multistage vacuum pumping and utilize a diffuse, low-power-density discharge for surface modification.
U.S. Pat. No. 5,314,539 by Brown et al. discloses an apparatus using multiple roller seals to generate a pressure interface and introduce a continuous strip of photographic substrate or similar material into a reduced-pressure (P.ltoreq.1 Torr) region for plasma treatment. A low-power RF discharge is used with air to modify the polymer substrate for improved wettability. The low discharge current of 20 to 200 milliamps for a 12.7-cm wide film suggests that a diffuse, low-power-density glow discharge is generated. The electrode shapes employed also require the low-pressure (P.ltoreq.1 Torr) operation. The sharp contours or bar shapes of the electrodes employed require that a discharge be ignited at modest voltages. Attempts to increase the discharge power density by significant increases in the discharge voltage would most likely result in plasma streamers and a poorly defined discharge. Because of the required low-pressure operation, multiple roller-seal stages are required, complicating the device's construction and maintenance.
U.S. Pat. No. 5,529,631 by Yoshikawa et al. discloses an apparatus using roller or gap seals to introduce a continuous sheet-like material into a high-pressure plasma treatment zone. The discharge is operated at essentially atmospheric pressure with helium gas and a few percent of a working gas as the discharge gas. Planar, dielectric-covered electrodes are excited with a sinusoidal signal of 50 Hz or greater. The system's use of significant amounts of helium would most likely compromise its cost effectiveness for large-volume, high-speed treatment of films or webs. Many applications for films require single-side treatment. This type of treatment would be difficult to obtain with these embodiments.
The above patent disclosures involve apparatus or processes that rely on the substrate being exposed directly to plasma species generated in the plasma-discharge zone. Polymer surfaces can also be modified by convecting plasma species and metastables out of the production zone and onto the polymer surface. This method is most effectively used at pressures below one Torr due to the rapid collisional recombination of ions that occurs at high gas pressures. The use of a microwave-generated plasma (f=2.45 GHz) to treat polymers films located downstream of the plasma source is discussed in Foerch et al., "Oxidation of polyethylene by remote plasma discharge: a comparison with alternative oxidation methods," J. Polym. Sci. A. 28, pp. 193-204 (1990), and Foerch et al., "A comparative study of the effects of remote nitrogen plasma, remote oxygen plasma, and corona discharge treatments on the surface properties of polyethylene," J. Adhesion Sci. Technology 5, pp.549-564 (1991). Surface levels of oxygen up to 30% and surface levels of nitrogen up to 40% were reported. The system employed operated at low pressure (P.about.1 Torr) and would be difficult to scale for wide films.
U.S. Pat. No. 4,937,094 by Doehler et al. discusses the use of a microwave- or RF-generated plasma to generate a high flux of metastable species which in turn are used to transfer energy to a remotely introduced precursor or etchant gas. Metastable species are generated by using a gas jet to convect active plasma species out of the microwave- or RF-discharge zone for a sufficiently long path that the ion species recombine to form metastables. Long-lived metastables are convected by the pressure differential, and are either used to collide with a remotely introduced gas with low-level ionization resulting or applied directly to a substrate material. The embodiment is directed primarily to amorphous-silicon deposition and etching techniques for semiconductor applications, and small-area treatments.
Atmospheric-pressure remote-plasma treatments have been reported by Frierich et al., "The improvement of adhesion of polyurethane-polypropylene composites by short time exposure of polypropylene to low and atmospheric pressure plasma," J. Adhesion Sci. Technology 9, pp. 575-598 (1995), using three different discharge methods: spark-jet plasma, arc-jet plasma, and corona-jet plasma. These discharges used low-frequency a.c., high-current d.c., and high-voltage a.c., respectively, to generate plasma discharges. Good lap-shear strength was obtained on laminated polypropylene-polyurethane samples with exposure times as short as 0.1 seconds. Due to gas-flow considerations and the electrode designs used, use of specialty gases and uniformity of treatment become concerns.
U.S. Pat. No. 5,458,856 by Maurepas et al. discloses a coaxial discharge used for the production of ozone, supplying a CO.sub.2 laser or the production of atmospheres for the nitriding of metals. The electrodes are excited using a low-frequency sinusoidal signal in the frequency range 20 kHz to 60 kHz. Ozone or excited metastables are convected radial out of the tubular gas passage formed by the coaxial geometry. The system is similar to the coaxial geometry of ozone generators used for water purification in Europe except with radial and azimuthal gas flow employed. The sharp edges and discontinuities of the design are not amicable to the generation of high-power, uniform-plasma discharges. No mention is made of treating web or film materials using the active-plasma species and metastables convected by means of sufficient gas flow.
The present invention provides improvements over the teachings of the prior art. Further aspects and advantages of this invention will become apparent from the detailed description which follows.